(1) Field of the Invention
The present invention relates to a silicon nitride sintered body having a high flexural strength at a high temperature and an excellent oxidation resistance, in which deviations of values of these characteristics are small, and a process for the preparation thereof. More particularly, the present invention relates to a silicon nitride sintered body in which the deviation of characteristics at a high temperature is controlled at the mass production, formation of a stain in a formed body is controlled and the oxidation resistance in a medium-temperature range (about 1000.degree. C.) such as at the time of idling of an engine is improved, and a process for the preparation of this silicon nitride sintered body.
(2) Description of the Related Art
A sintered body composed of silicon nitride has mainly covalent bonds as bonds of atoms and is excellent in such characteristics as strength, hardness, thermal stability and chemical stability. Therefore, applications of this sintered body to engineering ceramics, especially gas turbines as heat engines have been tried and developed.
It is expected that the operation temperature of a heat engine will be elevated above 1000.degree. C. with increase of the efficiency of the heat engine, and development of a material that can be used under this temperature condition is desired.
Various investigations have been made on the compositions of sintered bodies and sintering methods for obtaining sintered bodies excellent in high-temperature characteristics.
In connection with the composition, researches have been made especially on sintering aids. It is known that sintered bodies having good high-temperature characteristics can be obtained by addition of compounds of elements of the group IIIa of the periodic table and various oxides, but these sintering aids are still insufficient.
For example, U.S. Pat. No. 4,280,850 teaches that the Si.sub.3 N.sub.4 --Y.sub.2 O.sub.3 --SiO.sub.2 system is sintered to completely crystallize the grain boundary and improve the high-temperature strength. However, if the grain boundary is completely crystallized, since a vitreous phase is not present at all, the toughness (kic) is reduced and defects such as grinding scratches are easily formed, and a large deviation of the strength (flexural strength) is often caused.
U.S. Pat. No. 4,234,343 teaches that the Si.sub.3 N.sub.4 --M.sub.2 O.sub.3 --SiO.sub.2 system is sintered with Sc, Al, Cr, Ti, (Mg+Zn) or (Ni+Zr). An element having a small ion radius is selected as M of M.sub.2 O.sub.3. It is taught that if an element having a small ion radius is used, an Si.sub.3 N.sub.4 sintered body having excellent high-temperature strength and excellent oxidation resistance can be obtained and a disilicate (M.sub.2 Si.sub.2 O.sub.7) is formed in the grain boundary. However, the composition is SiO.sub.2 -rich and the amount of dispersed SiO.sub.2 is increased during sintering. Accordingly, the sintering method is limited, and only the HP sintering method or the sealed HIP method can be practically adopted.
Moreover, U.S. Pat. No. 4,388,414 discloses a process for preparing a thermally stable ceramic sintered body having a high oxidation resistance by using Y.sub.2 O.sub.3 in sintering the Si.sub.3 N.sub.4 --M.sub.2 O.sub.3 --SiO.sub.2 system, and it is taught that the weight increase by oxidation is 1.1 mg/cm.sup.2 in case of a grain boundary of wollastonite (K phase) or 0.25 mg/cm.sup.2 in case of a grain boundary of YAM (J phase). However, this sintered body is defective in that stains are formed.